1. Field of the Invention
This invention relates to safety apparatus and, more particularly, to a system for warning that a portion of a crane or other construction equipment has intercepted a boundary of a working space.
2. Description of the Prior Art
In the construction industry it is often necessary to operate cranes and excavators in areas having nearby danger zones such as high voltage power lines or other zones which must be kept free from intrusion by the crane and the crane boom. In the past, numerous accidents have occurred when the crane boom, the crane lifting lines or counterweights have come into contact with electrical power lines, and operators have been killed by electrocution. As a result industry and government regulators have recommended that a person be stationed on the ground near an operating crane to observe the clearance between the crane and electrical power lines and to give timely warning of danger for all operations where it is difficult for the operator to monitor the desired clearance by visual means. In practice, it has been found that a ground observer is not a reliable safety element and accidents have occurred when the ground observer was not present or when his attention was diverted.
Some prior art equipment has employed capacitive or inductive voltage pickup devices mounted on an insulating rod extending from the end of the crane and boom, however, such pickup devices are not very sensitive and the devices must be relatively close to electrical power lines for the crane operator to receive a warning signal. If the boom is moving toward the power lines rather rapidly the warning may be received too late and the operator may not be able to stop the boom in time to prevent a disaster. Also, such pickup devices are only useful near power lines and do not warn of other types of danger zones which may exist near the working area.